A technology for expanding a communication bandwidth is known as a link aggregation system (link aggregation). According to the link aggregation, an Ethernet (registered trademark) frame is broken up onto a plurality of physical links so as to be transmitted. The link aggregation was standardized by IEEE 802.3ad, and is defined by Chapter 43 of IEEE 802.3-2005 at present.
FIG. 12 illustrates a changed order of frames in a link aggregation system. Communication devices 101, 102 illustrated in FIG. 12 are connected to each other via links 103, 104. Square symbols including numerals illustrated in FIG. 12 indicate frames transmitted and received between the communication devices 101, 102. The numerals in the square symbols indicate the order of the frames.
The link aggregation includes an algorithm for distributing frames to the plural links 103, 104 on the basis of, e.g., a MAC (Media Access Control) address, an IP (Internet Protocol) address, a VLAN ID (Virtual Local Area Network IDentifier). If frames to be transmitted are simply distributed to the links 103, 104 alternately, the order of the frames may possibly be changed on the receiving side in some cases. Thus, Ethernet basically requires that the order of frames be not allowed to change.
Assume, e.g., that link lengths of the links 103, 104 are different from each other as illustrated in FIG. 12. In this case, the communication device 101 receives frames in an order of 1, 2, 3, 4, 5, and distributes the frames to the links 103, 104 alternately to transmit the frames. The communication device 102 receives the frames in an order of 2, 1, 4, 3, 5, though.
Thus, the MAC address, the IP address, the VLAN ID, etc. are made keys according to the link aggregation, so that the order of the frames is prevented from being changed at least in a same unit of communication (called a conversation). That is, frames of a same source address and a same destination address are distributed to the links as the conversation, so that the order of the frames is prevented from being changed between the frames belonging to the same conversation.
FIG. 13 is a diagram for explaining a conversation. Communication devices 111, 112 are connected to each other via links 113, 114. Terminals 115a, 115b communicate with each other via the communication devices 111, 112. Terminals 116a, 116b communicate with each other via the communication devices 111, 112. Square symbols including numerals illustrated in FIG. 13 indicate frames transmitted and received between the terminals 115a, 115b. Triangular symbols including numerals indicate frames transmitted and received between the terminals 116a, 116b. The numerals in the square and triangular symbols indicate the order of the frames.
The communication device 111 outputs the frames received from the terminals 115a and 116a to the links 113 and 114, respectively, on the basis of the MAC address, etc. The communication device 112 outputs the frames received from the links 113 and 114 to the terminals 115b and 116b, respectively, on the basis of the MAC address, etc. Thus, as illustrated in FIG. 13 by the square and triangular symbols including the numerals, the orders of the frames are maintained for the conversations on the terminals 115a, 115b and on the terminals 116a, 116b. 
According to the link aggregation, even if frames are transmitted via p-links, the communication bandwidth is not necessarily multiplied by n, e.g., in a case where volumes of traffic are large and very small on one link and on another link, respectively.
FIG. 14 illustrates an imbalance in the communication bandwidth. Each of portions illustrated in FIG. 14 which is a same as the corresponding one illustrated in FIG. 13 is given a same reference numeral, and its explanation is omitted. Assume, as illustrated in FIG. 14, that the volume of the traffic between the terminals 115a, 115b is larger than that between the terminals 116a, 116b. Even if the communication is carried out via the two links 113, 114 in this case, the communication bandwidth is not twice as broad as that in one-link case.
Thus, it is known to divide a plurality of frames of variable lengths into units of a specific size, and to distribute the units to respective ports aggregated as one broadband port so as to transmit the units, e.g., as disclosed in Japanese Laid-open Patent Publication No. 2005-252333. According to the above, the traffic is equally broken up to the plural ports having been aggregated, so that the communication bandwidth may be effectively used.
If units into which a frame is divided are put into a capsule on a transfer frame such as an Ethernet frame so as to be transmitted, there is a trade-off relation between transfer efficiency and a delay of the transfer frame.
FIG. 15 illustrates an exemplary frame format of an Ethernet frame. As illustrated in FIG. 15, the Ethernet frame has a header including a destination address, a source address and an Ethernet type. Further, the Ethernet frame has a payload and an FCS (Frame Check Sequence). The Ethernet frame and the previous frame have to be provided with an inter-frame gap of at least 20 bytes between those frames.
The maximum frame length of an Ethernet frame excluding the inter-frame gap is 1518 bytes. If units are included in the payload up to the 1500 byte maximum, transfer efficiency for transferring the units on the Ethernet frame including the inter-frame gap is 1500/1538=97.5 percent. Meanwhile, if units of only 100 bytes are included in the payload and transmitted, the transfer efficiency is 100/138=72.5 percent. That is, if the number of units included in the payload is small, the transfer efficiency decreases.
Thus, if units are included in the payload up to the 1500 byte maximum, then provided with an FCS and transmitted, the transfer efficiency of the Ethernet frame increases. As, however, the Ethernet frame is not transmitted until the units are included in the payload up to 1500 bytes, the frame transmission delays in case of no units to be transmitted.
As described above, if a frame divided into units is put into a capsule on a transfer frame such as an Ethernet frame so as to be transmitted, there is a trade-off relation between transfer efficiency and a frame delay of the transfer frame.
By the way, the frame indicates a unit of data transferred in the Data Link Layer (the Layer 2) of OSI reference model.